Our objective is to utilize L-forms of pneumococci and streptococci to understand the relationship of structure and function as it relates to the outer layers of the bacterial cell. The relationship of these layers to pathogenesis will be clarified by studying protoplasts and spheroplasts of groups D and A streptococci and their growth as L-phase variants or reversion to the coccal state. Reversion is to be studied with ferritin-labelled specific antibody directed against the cell membrane, mucopeptide and wall, followed by thin section electron micrography. Physiological studies of the L-form with comparison to the parent coccus in respect to synthesis of wall-precursors and carbohydrate metabolism will be undertaken. Induction survival and multiplication of L-forms in vivo requires that these bodies evade the host defense mechanisms. The interaction of L-forms with human polymorphonuclear leucocytes and their lysosomal cation proteins and the fate of L-forms following phagocytosis will be investigated.